Vacuum furnace

ABSTRACT

The downwardly open chamber of an electrically heated vacuum furnace for dental ceramics and the like may be sealed by a plate mounted on a hollow plunger. The plunger is axially movably received in a stationary cylinder, and a driving plug axially movable in the plunger is connected to a carrier for the object to be fired. A heating element in the top portion of the furnace chamber bounds a firing zone much hotter than the lower portion of the chamber so that a green body freshly introduced into the chamber may be protected from the highest furnace temperature while the furnace is being evacuated, and moved into the firing zone after the furnace is fully evacuated. The moving mechanism is pneumatically operated.

United States Patent Hintenberger 5] Feb. 8, 1972 [54] VACUUM FURNACE [2]] App]. No.: 38,721

I 30] Foreign Application Priority Data May 27, 1969 Germany ..P 19 26 874.7

3,128,326 4/1964 Hintenbergerl.....l. ..f.

FOREIGN PATENTS OR APPLICATIONS 1,160,777 1/1964 Germany ..13/31 Primary ExaminerA. D. Pellinen Attorney-Kurt Kelman [57] ABSTRACT The downwardly open chamber of an electrically heated vacuum furnace for dental ceramics and the like may be sealed by a plate mounted on a hollow plungen The plunger is axially movably received in a stationary cylinder, and a driving plug axially movable in the plunger is connected to a carrier for the object to be fired. A heating element in the top portion of the furnace chamber bounds a firing zone much hotter than the lower portion of the chamber so that a green body freshly introduced into the chamber may be protected from the highest furnace temperature while the furnace is being evacuated, and moved into the firing zone after the furnace is fully evacuated. The moving mechanism is pneumatically operated.

3Claims, 2Drawing Figures mm a an SHEET 1 [IF 2 Fig.7

7 PRIOR ART lnvenlor lam, HM/TENBGKEA 7 RGEUT VACUUM FURNACE This invention relates to vacuum fumaces, and particularly to a furnace suitable for firing dental porcelain.

More specifically, this invention is concerned with an improvement in an electrically heated furnace whose chamber is downwardly open and can be sealed by a plate connected to the work carrier as disclosed in my German Pat. No. 1,160,777 or my U.S. Pat. No. 3,128,326. A green body of ceramic material may be moved for preheating into the bottom portion of the furnace chamber on the carrier, and thereafter moved into the firing zone at the top of the chamber, the chamber opening being sealed simultaneously.

It is not permissible to expose the preheated ceramic material to the full fumace heat before the vacuum is established, and it is necessary to lower the temperature in the firing zone of the known furnace whenever a fired object is removed and a new object is to be charged. The cyclic temperature variation is harmful to the furnace, and particularly to the heating element which is ill suited to withstand cyclic thermal stresses, and the life of the heating element in the known apparatus is relatively short.

It is the primary object of this invention to provide a vacuum furnace which retains the advantages of the aforedescribed furnace, but avoids its shortcomings. More specifically, it is an object of this invention to provide a furnace of the type described in which the heating element may be kept at practically constant temperature during a sequence of individual firing runs.

With this object and others in view, as will hereinafter become apparent, the furnace of the invention has a shell whose chamber may be connected with the ambient atmosphere by an opening. A heating device in the chamber defines a firing zone whose temperature is higher than that of the remainder of the chamber when the heating device operates. A support is mounted for movement outside the chamber toward and away from an operating position adjacent the opening. A sealing device responds to movement of the support into the operating position for sealing the chamber opening, whereupon air may be withdrawn from the chamber for evacuating the same. A transfer mechanism is interposed between the support and a work carrier for the object to be fired and moves the carrier into the chamber when the support moves toward the operative position. The carrier may be transferred in the chamber between the firing zone and the cooler remainder of the chamber by the transfer mechanism while the support is in its operating position.

Other features, additional objects, and many of the attendant advantages of this invention will readily become apparent as the same becomes better understood by reference to the following detailed description of a preferred embodiment when considered in connection with the appended drawing in which:

FIG. 1 shows a known dental vacuum furnace arrangement in front elevation, and partly in section; and

FIG. 2 shows a portion of the furnace arrangement of FIG. 1 incorporating the features of the invention in rear elevational section on a much larger scale.

Referring now to the drawing in detail, and initially to FIG. I, there is seen a generally cubical steel shell 4 which is lined with fire bricks 3. The bricks enclose the furnace chamber 1 whose bottom is open and which communicates with the ambient atmosphere through a circular opening 8 in the bottom wall 17 of the shell 4 in the illustrated condition of the apparatus. An annular heating element 2 bounds the topmost portion of the chamber 1, as is described in more detail in my simultaneously filed application Ser. No. 38,720, for a FUR- NACE CHAMBER.

A vacuum line 7 leads into the chamber through the top of the shell 4 and a bore in a brick 3. Four steel columns 5 spacedly attach the shell 4 to a base 6 which carries the furnace controls and indicating instruments, conventional in themselves, and not directly relevant to this invention.

The base 6 also carries the operating mechanism 9 for moving workpieces into and out of the chamber 1 and for sealing the charged chamber with which this invention is more specifically concerned. A ceramic carrier 15 is releasably mounted atop the mechanism 9, and carries a ceramic tooth 14 which is ready to be fired.

The mechanism 9 according to the invention is shown in detail in FIG. 2. An upright cylinder 11 is fixedly mounted in the base 6 in axial alignment with the opening 8. Its top is open, and its bottom is connected to a supply line 12 for compressed air. A hollow plunger or piston 10 is axially slidable in the cavity of the cylinder 11 in telescoping engagement. The interior of the plunger 10 communicates with the cylinder cavity through a throttling orifice 18 in the bottom wall 25 of the plunger. The top end of the plunger 10 projects from the cylinder 11 in the illustrated lowermost position of the plunger and carries a circular radial plane or sealing plate 10'. A flat ring 16 of asbestos composition is coaxially set into the top face of the sealing plate 10' for engagement with the bottom wall 17 about the opening 8 when the plunger 10 moves outward of the cylinder 11.

The bore of the plunger 10 is upwardly open and receives a telescoping transfer mechanism mainly consisting of a cylindrical driver 13 of solid metal axially slidable in the plunger 10 in sealing engagement with the latter, a rod 22 of reduced cross section extending upward from the driver 13 and out of the plunger 10, and a radial flange or mounting plate 23 on the top end of the rod 22. The carrier 15, not itself seen in FIG. 2 is normally secured to the mounting plate 23 by a threaded fastener, only a threaded central opening in the plate 23 being seen in FIG. 2.

A bleeder valve 19 controls a radial opening in the cylinder 11 which is covered by the plunger 10 in the illustrated position of the apparatus. A collar 21 fixed to the plunger 10 under the sealing plate 10 is threadedly connected to a long radial handle 20.

The aforedescribed apparatus is operated as follows:

When compressed air is admitted to the cavity of the cylinder 11 from the line 12, which is equipped with conventional control valves, not shown, the plunger 10 is expelled from the cylinder and moves toward the opening 8. The movement is slowed as soon as the bottom wall 25 clears the bleeder valve 19, and a part of the supplied air can escape into the atmosphere. The air pressure is not sufficient to displace the heavy driver 13, rod 22, and mounting plate 23. The entire operating mechanism 9, as far as visible in FIG. 1, rises as a unit until the ring 16 sealingly engages the bottom wall 17, whereby the tooth 14 is enclosed in the lowermost portion of the chamber 1.

The nonillustrated vacuum pump at the end of the line 7 may then be actuated to draw the air from the chamber 1. During the evacuation period, the tooth remains cool enough so that air occluded in the ceramic mass can escape and is not trapped in partly molten material. After the desired vacuum is reached, the air pressure from the line 12 is increased or the bleeder valve is moved toward the closed position to raise the driver 13 whose movement is transmitted to the work carrier 15 by the rod 22 and the mounting plate 23. The tooth 14 may be raised stepwise or continuously to bring it ultimately into the firing zone bounded by the heating element 2.

When firing is completed, the driver 13 and associated elements may be lowered, the vacuum broken, and the plunger 10 and sealing plate 10 lowered to the illustrated inoperative position by venting the line 12 to atmosphere. The handle 20 has been found convenient for manually expediting the rise and descent of the plunger 10 and the discharge of the carrier 15 and of its load from the chamber 1. A new cycle can begin. The current to the heating element may be kept cbnstant throughout a sequence of operating cycles.

It is usually convenient in a small unit of the type described to provide compressed air and vacuum from a single pump, not shown, whose intake and discharge ends are respectively connected to the vacuum line 17 and the pressure line 12. The position of the transfer mechanism, which is hidden from view by the sealed chamber 1, may be indicated in a conventional manner, if so desired, as by a limit switch, not shown, which is arranged in the plunger at the level of the collar 21, and is actuated by the top edge of the driver 13 to light a pilot lamp or produce another suitable signal.

lt'is preferred to provide the bleeder valve 19 and the nonillustrated control valves in the vacuum line 17 and compressed air line 12 with solenoid actuators and return springs so that they can be operated remotely by pushbuttons on the base 6, as is conventional in itself.

It should be understood, of course, that the foregoing disclosure relates only to a preferred embodiment of the invention, and that it is intended to cover all changes and modifications in the example of the invention chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the attached claims.

I claim:

1. In a furnace for firing a ceramic object, which comprises a shell defining a chamber therein, the shell having a closed top and a bottom opening connecting the chamber with the ambient atmosphere, a firing zone adjacent the closed top, a sealing plate mounted outside the chamber in alignment with the chamber opening for pneumatically actuatable movement into sealing engagement with the chamber opening, a pneumatic cylinder receiving compressed air, a piston afiixed to the sealing plate and glidable in the cylinder, the piston being actuated under the pressure of the compressed air to move the sealing plate into said sealing engagement, evacuating means for withdrawing air from the chamber when said opening is sealed, and a carrier for the object between the sealing plate and the chamber opening, the improvement of the sealing plate piston defining an axial bore, a piston glidable in the sealing plate piston bore and afiixed to the carrier, and a constricted orifice in the sealing plate piston for admitting a controlled amount of the compressed air into the axial bore to move the carrier piston in respect of the sealing plate piston.

2. In the furnace of claim 1, the sealing plate piston including a bottom closure closing the axial bore, the constricted orifice being defined in the bottom closure.

3. A furnace as set forth in claim 2, further comprising an adjustable bleeder valve connecting said axial bore with said atmosphere. 

1. In a furnace for firing a ceramic object, which comprises a shell defining a chamber therein, the shell having a closed top and a bottom opening connecting the chamber with the ambient atmosphere, a firing zone adjacent the closed top, a sealing plate mounted outside the chamber in alignment with the chamber opening for pneumatically actuatable movement into sealing engagement with the chamber opening, a pneumatic cylinder receiving compressed air, a piston affixed to the sealing plate and glidable in the cylinder, the piston being actuated under the pressure of the compressed air to move the sealing plate into said sealing engagement, evacuating means for withdrawing air from the chamber when said opening is sealed, and a carrier for the object between the sealing plate and the chamber opening, the improvement of the sealing plate piston defining an axial bore, a piston glidable in the sealing plate piston bore and affixed to the carrier, and a constricted orifice in the sealing plate piston for admitting a controlled amount of the compressed air into the axial bore to move the carrier piston in respect of the sealing plate piston.
 2. In the furnace of claim 1, the sealing plate piston including a bottom closure closing the axial bore, the constricted orifice being defined in the bottom closure.
 3. A furnace as set forth in claim 2, further comprising an adjustable bleeder valve connecting said axial bore with said atmosphere. 